[lld] r248106 - COFF: Fix race condition.

[Rui Ueyama via llvm-commits]

I mean I would do that if there's a portable way in C++11 to get the
current thread ID as a small integer. If we have such function, we could
write like this.

  thread_local uint64_t NextID = 1;

 ...

  ID = (getThreadID() << 32) + NextID++;

But I don't think such function exists.

On Mon, Sep 21, 2015 at 3:49 PM, Rui Ueyama <ruiu at google.com> wrote:

> On Mon, Sep 21, 2015 at 3:35 PM, Sean Silva <chisophugis at gmail.com> wrote:
>
>>
>>
>> On Mon, Sep 21, 2015 at 8:20 AM, Rui Ueyama <ruiu at google.com> wrote:
>>
>>> On Sun, Sep 20, 2015 at 6:42 PM, Rui Ueyama <ruiu at google.com> wrote:
>>>
>>>> We can, but I think we don't have to, because I didn't see any
>>>> measurable difference between this code and the old racy one.
>>>>
>>> ... in terms of speed.
>>>
>>
>> On x86 at a hardware level a similar amount of synchronization is
>> required for both cases, so this isn't surprising (regardless of whether
>> the access is atomic or not, the cache coherence mechanisms kick in).
>> Simply replacing a non-atomic access with an atomic access is not enough.
>> From inspection of the code, there seems to be quite a bit of work being
>> done between increments of this value, so it's probably not going to
>> matter, but if you someday change this to use fine-grained parallelism so
>> that it scales linearly with # cores (it is highly parallelizable, so this
>> should be possible), it will probably be necessary to factor this in.
>>
>> For reference, I have attached a simple test program that shows the
>> relative impact of the different choices.
>>
>> Sean:~/tmp % clang++ -std=c++11 -DNUM_THREADS=12
>> -DNUM_OPS_PER_THREAD=5000000 -O3 testsynchronization.cpp && ./a.out
>>
>> Per thread, non-atomic: 1.00x (baseline)
>>
>> Per-thread, atomic:     3.61x slower than baseline
>>
>> Shared, non-atomic:     29.47x slower than baseline
>>
>> Shared, atomic:         112.72x slower than baseline
>>
>> This is on a Xeon E5-2697 v2 (12 physical cores).
>>
>> As you can see, the effect of making the access non-atomic is very small
>> compared to the effect of avoiding cache coherence overhead (3-4x vs. 30x).
>>
>
> Thank you for the interesting numbers. Shared atomic variable are indeed
> much slower than non-shared, non-atomic variables when you have a lot of
> concurrent writes. If I'm really trying to get until the last drop from
> multi-cores, I'd do as you suggested so that threads share almost nothing.
> I didn't do that simply because the cost of updating the atomic counter is
> in practice negligible in this code because we are doing so many things
> other than updating the atomic variable. I don't generally optimize unless
> doing that makes measurable performance difference.
>
>
>> CC'ing Dmitry who may have more insight to give.
>>
>> -- Sean Silva
>>
>>
>>>
>>>
>>>
>>>> 2015/09/20 18:32 "Sean Silva" <chisophugis at gmail.com>:
>>>>
>>>> It seems unfortunate for us to need any synchronization at all just to
>>>>> generate a unique ID. Can we just have a bunch of counters, one per thread,
>>>>> then initialize them to `ThreadIndex << 50` or something? (make sure they
>>>>> are each on their own cache line)
>>>>>
>>>>> -- Sean Silva
>>>>>
>>>>> On Sat, Sep 19, 2015 at 6:44 PM, Rui Ueyama via llvm-commits <
>>>>> llvm-commits at lists.llvm.org> wrote:
>>>>>
>>>>>> Author: ruiu
>>>>>> Date: Sat Sep 19 20:44:44 2015
>>>>>> New Revision: 248106
>>>>>>
>>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=248106&view=rev
>>>>>> Log:
>>>>>> COFF: Fix race condition.
>>>>>>
>>>>>> NextID is updated inside parallel_for_each, so it needs mutual
>>>>>> exclusion.
>>>>>>
>>>>>> Modified:
>>>>>>     lld/trunk/COFF/ICF.cpp
>>>>>>
>>>>>> Modified: lld/trunk/COFF/ICF.cpp
>>>>>> URL:
>>>>>> http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/ICF.cpp?rev=248106&r1=248105&r2=248106&view=diff
>>>>>>
>>>>>> ==============================================================================
>>>>>> --- lld/trunk/COFF/ICF.cpp (original)
>>>>>> +++ lld/trunk/COFF/ICF.cpp Sat Sep 19 20:44:44 2015
>>>>>> @@ -73,7 +73,7 @@ private:
>>>>>>    bool forEachGroup(std::vector<SectionChunk *> &Chunks, Comparator
>>>>>> Eq);
>>>>>>    bool partition(ChunkIterator Begin, ChunkIterator End, Comparator
>>>>>> Eq);
>>>>>>
>>>>>> -  uint64_t NextID = 1;
>>>>>> +  std::atomic<uint64_t> NextID = { 1 };
>>>>>>  };
>>>>>>
>>>>>>  // Entry point to ICF.
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> llvm-commits mailing list
>>>>>> llvm-commits at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>>>
>>>>>
>>>>>
>>>
>>
>
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